Not applicable.
The present invention generally relates to methods and systems for managing/dissipating heat generated by electronic devices. More particularly, this invention relates to an encapsulation material for electronic devices, in which the encapsulation material contains a phase change material that absorbs heat generated by transient thermal events.
Heat generated by electronic devices during normal operation can cause overheating and device failure if not conducted away. While steady-state heat must be removed from the device and dissipated to the surrounding environment to prevent overheating, transient energy can be absorbed by the device and any surrounding packaging material and then dissipated to the environment more slowly. An example of a packaging material is a potting compound used to seal and protect a circuit device mounted on a circuit board within an open or closed case. Transient thermal energy can be absorbed with the use of packaging materials by two mechanisms. The first and most commonly used is the energy absorbed by the packaging material due to its inherent specific heat. The second and less commonly used mechanism is to absorb thermal energy by using the latent heat of the packaging material as the material undergoes a phase change. The most common phase change materials are of three types: solid-to-liquid, liquid-to-gas, and solid-to-solid phase change. Each of these types of phase change materials has disadvantages for electronic applications. Though solid-to-liquid phase change materials are capable of absorbing relatively large amounts of thermal energy, costly sealed containment is required to prevent loss of the material when in the liquid state. Liquid-to-gas phase change materials are capable of absorbing even greater amounts of thermal energy, but the cost of containing the gas phase is typically higher than that for solid-to-liquid materials. Alternatively, the heat dissipation system must allow for replenishing the phase change material. In contrast to the preceding materials, solid-to-solid phase change materials do not require special or complicated containment structures. However, the energy absorption of these materials is considerably less than solid-to-liquid or liquid-to-gas phase change materials. Another shortcoming is that solid-to-solid phase change materials are unable to provide an effective barrier to liquid and ionic contamination when used to encapsulate or pot electronic modules.
It can be appreciated that for a phase change material to be suitable for a wide range of applications in electronic products, the material should not only have relatively high thermal capacitance during the phase change, but should also be relatively low cost and self-containing, i.e., not require any sealing or special containment over the operating temperature range of the device being packaged. Additional desirable properties for electronic applications include a high thermal conductivity to quickly transport heat away from a heat-generating component, and the capability of customizing the temperature at which the phase change occurs. The capability to be used as a potting compound is also desirable for many applications, requiring that the phase change material have a high electrical resistance for direct contact with an electrical circuit and the ability to stop liquid and ionic intrusion. As a potting material, the material should also preferably have low ionic content for corrosion resistance, a low elastic modulus over the intended temperature range in order to minimize stress on the electrical components during thermal cycling, and good adhesion and minimal material property changes after long term exposure to the operating environment. While phase change materials exist that have several or more of these preferred characteristics, it would be desirable if a phase change material existed that exhibited each of the characteristics to the degree necessary to be useful in a wide variety of applications, and particularly as an encapsulation or potting material for an electronic module.
The present invention provides an encapsulation material suitable for dissipating heat generated by an electronic module, such as by directly contacting a heat-generating power device or contacting a heat sink of a heat-generating power device. The encapsulation material comprises phase change particles dispersed in a polymer matrix material. The particles preferably comprise a metallic alloy encapsulated by a dielectric coating so the particles are electrically insulated from each other. The encapsulation material may further comprise dielectric particles dispersed in the polymer matrix material for the purpose of increasing the thermal conductivity of the encapsulation material. Alternatively or in addition, the dielectric coating on the particles may comprise dielectric particles that are dispersed in a dielectric matrix, again with the preferred effect of increasing the thermal conductivity of the encapsulation material.
According to the invention, the metallic alloy particles enable the encapsulation material to perform as a solid-to-liquid phase change material having characteristics highly desirable for use in electronic applications. For example, the encapsulation material is thermally conductive and has relatively high thermal capacitance during the phase change as a result of the presence of the metallic alloy particles, yet can be used in the form of a semisolid gel capable of containing the molten metallic particles so as not to require complicated sealing or containment over the operating temperature range of the particular electronic application. As a result of the polymer matrix material, the encapsulation material of this invention is electrically nonconductive and capable of stopping liquid and ionic intrusion, and therefore is suitable as a potting material. In addition, preferred polymer matrix materials have low ionic content for corrosion resistance, a low modulus to minimize thermally-induced mechanical stresses, good adhesion properties, and stable material properties even after long exposures within operating environments typically required of electronic devices.
Other objects and advantages of this invention will be better appreciated from the following detailed description.